The preparation of thermo-crosslinkable and/or thermoplastic elastomer blends, in particular of vulcanizable rubber blends, can be effected batchwise or continuously, for example on a calender or in an internal mixer. In both cases, the duration of the mixing process is important, so that only moderate throughputs can be obtained.
However, the use of an internal mixer has the disadvantage that, due to the applied speed of rotation, the crude rubber blend is so strongly heated that for many mixture formulations no crosslinking reagents can be added. Therefore, it is often necessary either
to arrange a calender downstream of said internal mixer and to add sulfur and accelerators to the crude blend removed from the internal mixer on said calender only;
or alternatively
to prepare a preblend without sulfur and accelerator in a first mixing passage through said internal mixer, and thereafter to prepare the final blend with addition of sulfur and accelerator in a second mixing passage through said internal mixer.
So far, the attempts for achieving a continuous mixing in mixing extruders has failed for the abovementioned compulsion of observing this order. Calculations show that a spindle length of the order of 40 times diameter would be necessary (D being the spindle diameter). This is technically difficult to realize and economically unattractive.
Furthermore, the use of plasticized elastomers or the addition of large quantities of plasticizers is critical since the shearing forces produceable in the mixture are no longer sufficient for a regular mixing.